Fix light and dark UI regressions

[Leoncl2025]

Summary

• restore the native Windows palette and upstream current-line color in Light mode
• split Light and Dark stylesheets so native Light menus and dialogs keep their platform rendering
• replace thick Dark menu separator bands with one-pixel dividers
• fully theme Dark save prompts, including the body, footer, text, and button sizing
• add palette, editor, menu, and message-box regression coverage

Validation

• Release build with Visual Studio 2022 / MSVC 19.44 and Qt 6.5.3
• AppearanceManagerTests
• EditorAppearanceTests
• CompareEngineTests
• focused CompareSessionTests: Light native controls, Dark menu separators, Dark message boxes, and inactive Dark tabs